Due to their stability, relatively low toxicity, compatibility with numerous substances, and low vaporization points, short chain fluorocarbons have been found to have utility or potential utility in industry for many purposes. Such short chain fluorocarbons (about 1 to about 5 carbon atoms) have, for example, been used as propellants, refrigerants, and solvents.
It has been asserted that certain fluorocarbons, especially chlorofluorocarbons, unfortunately may be hazardous to the environment, especially to the ozone layer. Further, impurities in certain fluorocarbons have been found to be troublesome. Unsaturated fluorocarbons are especially undesirable since many of them are toxic and are also often unstable. Such unstable unsaturated fluorocarbons can decompose into even more undesirable products which can be corrosive. It is therefore especially important for certain applications that fluorocarbons be essentially free (e.g. less than 100 ppm) of unsaturated species. There has therefore been a concerted effort to develop new and purer fluorocarbons which are viewed as being more environmentally friendly.
With respect to some fluorocarbons, it has been found possible to reduce the quantity of unsaturated species by chlorination, e.g., as described in U.S. Pat. Nos. 5,190,626 and 5,336,377. Unfortunately, the outcome of such a chlorination technique is not predictable from one fluorocarbon to another, especially when hydrogen is present in the fluorocarbon being purified. This is because hydrogen is frequently replaced by chlorine which reduces yield of the desired product and results in yet further impurities. In certain applications, such fluorocarbons must be especially pure, e.g., when they are used in particularly sensitive areas such as for refrigerants in air conditioners in confined areas.
It has been found that, 1,1,1,3,3,3-hexafluoropropane (HFC-236fa), can be used as a replacement for the fluorocarbon CCl2F2CClF2(CFC-114), used as a refrigerant. HFC-236fa has found particular use as a refrigerant in a closed environment. HFC-236fa can be prepared by numerous methods, e.g., as described in U.S. Pats. Nos. 5,395,997; 5,414,165; and World Patent Application WO96/15085-A1. 1,1,1,3,3,3-hexafluoropropane might be an excellent replacement for CFC-114, except that pure 1,1,1,3,3,3-hexafluoropropane has not been obtainable by known methods of preparation. This due to the fact that most known methods for the preparation of 1,1,1,3,3,3-hexafluoropropane result in residual impurities of other fluorocarbons. Unfortunately, some of the fluorocarbon impurities form low boiling azeotropes with 1,1,1,3,3,3-hexafluoropropane or are close boiling with 1,1,1,3,3,3-hexafluoropropane, which prevents separation by conventional distillation methods (“distillation inseparable mixtures”). Such distillation inseparable mixtures are obtained when an attempt is made to purify 1,1,1,3,3,3-hexafluoropropane by distillation from the reaction mixture in which it is made. This is especially troublesome since some of the impurities which form azeotropes or close boiling mixtures are unsaturated and cannot be tolerated to any significant extent in refrigerants in certain applications. Examples of such undesirable unsaturated fluorocarbon impurities are 1,1,1,3,3-pentafluoro-2-chloropropene obtained by the liquid phase reaction described in U.S. Pat. No. 5,395,997 and C3HCl2F3 obtained by the vapor phase reaction described in U.S. Pat. No. 5,414,165.
It has been proposed in U.S. Pat. Nos. 5,856,595 and 6,274,779 B1 to purify HFC-236fa from distillation inseparable mixtures of 1,1,1,3,3,3-hexafluoropropane with at least one unsaturated fluorocarbon to obtain a 1,1,1,3,3,3-hexafluoropropane product of greater than 99.9 weight percent purity containing less than 100 parts per million of unsaturated fluorocarbons by a batch, liquid phase method that comprises:
a) reacting the mixture with chlorine to saturate the unsaturated fluorocarbons in a reacted mixture,
b) distilling the reacted mixture to obtain a 1,1,1,3,3,3-hexafluoropropane, and
c) removing residual HCl and chlorine from the 1,1,1,3,3,3-hexafluoropropane at any point in the method subsequent to reacting the mixture with chlorine to saturate the unsaturated fluorocarbon.
In the method of those patents, it is taught that the reaction of the mixture with chlorine is preferably conducted in the presence of ultraviolet light (UV). However, the patents do not disclose any specific UV photochlorination equipment suitable for use in accomplishing the objective of those patents, nor do patents disclose any photochlorination reaction equipment suitable for conducting the purification reaction in a continuous and vapor phase reaction while still being able to obtain a purified HFC-236fa product containing <100 ppm fluorocarbon unsaturates. It is highly desirable that such a continuous process be obtainable that produces HFC-236fa with <1000 ppm, preferably <500 ppm, and more preferably <100 ppm fluorocarbon unsaturates.